This invention relates to a method of manufacturing an oriented silicon steel sheet having improved magnetic characteristics and, more particularly, to an improved cold rolling process which enables improvements in productivity and magnetic characteristics of the sheet.
Among magnetic characteristics of oriented silicon steel sheets, a high magnetic flux density and a low core loss are important. Recent progress of manufacture techniques has made it possible to obtain, for example, a steel sheet having a magnetic flux density B.sub.s (value at a magnetizing force of 800 A/m) of 1.92 T with respect to a thickness of 0.23 mm and also to manufacture, on an industrial scale, an improved product having a core loss characteristic W.sub.17/50 (a value under a fully magnetized condition: 1.7 T at 50 Hz) of 0.90 w/kg.
Sheet having such an improved magnetic characteristic has a crystalline structure in which &lt;001&gt;directions corresponding to an axis of easy magnetization of iron are uniformly aligned with the direction of rolling of the steel sheet. Such an texture is formed during finishing annealing in an oriented silicon steel sheet manufacturing process by secondary recrystallization in which crystal grains having a (110) [001]direction called the Goss orientation are grown with priority into giant grains. As fundamental requirements for sufficiently growing secondary recrystallized grains, the existence of an inhibitor for limiting the growth of crystal grains having undesirable directions other than the (110) [001]direction in the secondary recrystallization process and the formation of a primary recrystallized texture suitable for developing secondary recrystallized grains of (110) [001]direction are required, as is well known.
A fine precipitate of MnS, MnSe, AlN or the like is ordinarily utilized as an inhibitor. Also, enhancing the effect of the inhibitor by adding a grain boundary segregation type component such as Sb or Sn to the inhibitor has been practiced, as disclosed in Japanese Patent Publication Nos. 51-13469 and 54-32412.
On the other hand, various means have conventionally been used in the steps of hot rolling and cold rolling to form a suitable primary recrystallized texture. For example, with respect to a cold rolling method using AlN as an inhibitor, it has been considered that processing the steel by the thermal effect of warm rolling or inter-pass aging during cold rolling as disclosed in Japanese Patent Publication No. 50-26493, 54-13846 or 54-29182 is particularly effective. This kind of technique is based on the idea of forming a suitable texture by using the mutual effect between solid solutions N and C and dislocations in the steel so that the mechanism of deformation of the material during rolling is changed.
However, the above-described methods of the prior art are rather disadvantageous in terms of productivity and do not always ensure the effect of obtaining an improved magnetic characteristic with stability. For example, it is still difficult to carry out warm rolling on an industrial scale for technical reasons. With respect to inter-pass aging, it is a common practice to heat-treat the steel in a coiled state a plurality of times with a one-stand reverse rolling mill, because it is very difficult to heat-treat the steel uniformly throughout the overall coil length.
A technique of using a tandem rolling mill having a plurality of rolling stands to improve productivity has recently come into popular use. Rolling using a tandem rolling mill, unlike rolling using a reverse rolling mill, requires matching of rolling ratio and the rolling speeds between preceding rolling stand and following rolling stand Naturally it mainly causes compressed deformation by compression, not by tension. The rolling deformation mechanism of this type of rolling thus differs greatly from those of other conventional rolling methods, and the effect of the conventional aging method is therefore unsatisfactory. In this situation tandem rolling for a high-magnetic-flux-density silicon steel sheet containing Al is particularly difficult. Moreover, because of characteristics of tandem rolling, the production efficiency is considerably reduced if aging is repeatedly effected, and it is undesirable to effect aging a plurality of times for the purpose of improving the productivity as in conventional methods.